Process for the continuous production of sub-micron two-dimensional materials such as graphene

ABSTRACT

A system and a method of continuously separating submicron thickness laminar solid particles from a solid suspension, segregating the suspension into a submicron thickness particle fraction suspension and a residual particle fraction suspension, the method comprising the steps of; providing a continuous centrifuge apparatus; providing a suspension of submicron thickness laminar solid particles in a solid suspension; wherein the solid suspension comprises the submicron thickness solid particles in a liquid continuous phase; separating the solid suspension in the apparatus.

The project leading to this application has received funding from theEuropean Union's Horizon 2020 research and innovation programme undergrant agreement No 646155.

FIELD OF THE INVENTION

The invention relates to a process, preferably a continuous separationprocess, for producing dispersions of atomically thin 2-dimensionalmaterials, preferably up to 100 nm in thickness most preferably in therange one atom thick up to 30 layers. In particular, the inventionrelates to a scalable continuous process for separating 2-dimensionalmaterials by size, including by thickness, for example graphene,molybdenum disulphide or boron nitride, to assist industrial production.

BACKGROUND TO THE INVENTION

Graphene is a 2-dimensional allotrope of carbon, consisting of a fewatoms thickness in a hexagonal structure. Graphite, a widely usedmineral, is effectively a three-dimensional form of graphene, withmultiple layers bound together by van der Waals forces. Graphene hasattracted a lot of interest since it was first isolated in 2004. Thenovel mechanical, thermal and electrical properties of the material areanticipated to enable a number of uses, and there is a very activeresearch community developing applications for graphene. Graphene hasbeen produced on a laboratory scale sufficient for experimentalanalysis, but production in commercial quantities is still a developingarea. Other single layered structures such as molybdenum disulphide orboron nitride are expected to exhibit similarly interesting propertiesin the nanotechnology field.

Min Yi and Zhigang Shen, (A review on mechanical exfoliation for thescalable production of graphene, Journal of Materials Chemistry, A,2015, 3, 11700) provide an overview of the state of the art regardinggraphene production.

Large-scale production of graphene at a low cost has been demonstratedto be possible using top-down techniques, whereby graphene is producedthrough the direct exfoliation of graphite in a liquid phase. Thestarting material for this is three-dimensional graphite, which isseparated by mechanical and/or chemical means to produce graphene sheetsa few atoms thick.

Prof. Jonathan Coleman's group at Trinity College Dublin developed aproduction route for graphene by sonication-assisted liquid-phaseexfoliation of graphite in 2008. Starting with graphite powder dispersedin specific organic solvents, followed by sonication and centrifugation,they obtained a graphene dispersion. This method of producing grapheneseemed relatively easy. The main shortcoming of this method is theextremely low graphene concentration (around 0.01 mg/mL), which is farfrom practical application.

More recently, fluid-dynamics based methods have emerged for graphiteexfoliation. These are based on mixing graphite in a powder or flakeform with a fluid to form a suspension. The fluid can then be subjectedto turbulent or viscous forces which apply shear stress to the suspendedparticles. The shear stress exfoliates graphene platelets from thegraphite, and these remain suspended in the fluid. Usually the fluid iseither a solvent, or a surfactant mixture which must be removed from thefinished product.

Removing the graphene platelets from the fluid is usually carried out bycentrifuging the suspension in a desk top or laboratory centrifuge for45 minutes or more. The larger particles form a sediment and thesupernatant fluid is then further processed to extract the desiredplatelets from the liquid. In order to select graphene nano-plateletswith only a few atomic layers thickness, the process typically needs tobe repeated several times,

US Patent application 20150283482 describes a method of sorting2-dimensional materials by thickness using density gradientultracentrifugation (DGU). In one example, molybdenum disulphide wasultracentrifuged at 32 krpm for 24 hours. During DGU, the differences inthe buoyant density of the two-dimensional nanomaterials and that of thesurrounding medium drive the two-dimensional nanomaterials to theirrespective isopycnic points, where the buoyant density of a particularnanomaterial crystallite matches that of the surrounding medium.

International Patent Application WO2014001519 provides a process forexfoliating a 3-dimensional layered material to produce a 2-dimensionalmaterial, including the step of centrifuging at 2000 rpm for 45 minutes.

In “Size selection of dispersed, exfoliated graphene flakes bycontrolled centrifugation”, Umar Khan et al, June 2011 describe aprocess of centrifuging an exfoliated graphite suspension.Centrifugation at high rates results in small flakes being dispersed butlarger ones sedimenting out. This sediment can be collected andredispersed. Centrifugation at a lower rate then results in a dispersionof slightly larger flakes and the rejection of the rest. Repeating thisprocedure, a number of times results in the separation of the originaldispersion into a number of fractions each with different mean flakelength, in this case from ˜1 μm to ˜3.5 μm.

Graphite, the multi-layered crystalline form of graphene has a densityover twice that of water and will readily settle out of suspension.However, once a layer of graphene is coated with surfactant, thecombination of a layer with the surfactant on each face reduces thedensity. For single layer graphene, the density is close to 1.1 g/cc,only 10% above that of water. For each additional graphene layer thedensity increases. Because nano-platelets coated with surfactant behaveas a colloid, the particles can remain in suspension for days or weeks.

Centrifugation can be used to separate a fine suspended solid from aliquid. There are various types of centrifuge. A laboratory centrifugeis the obvious choice, as the centrifuge fluid properties can becarefully controlled and the time and rotational velocity of thecentrifuge can be selected to obtain the desired degree of separation.

The speed of separation also depends on particle size, as drag force oneach particle depends on particle size and the viscosity of the fluidthey are suspended in. Under a constant accelerating force, eachparticle in suspension reaches a terminal velocity at which it migratesthrough the fluid.

Graphene produced by different initial processes will have differentflake sizes and different distributions of flake thickness.Centrifugation can sort the graphene flakes into different thicknessesand/or different sizes of nano-platelets, but can take considerabletime. In US patent application 20150283482 for example some of thecentrifugation examples take 24 hours to complete, producing only a fewml of concentrated suspension and this needs to be repeated in manyinstances to obtain the final product.

After centrifugation, a filtration step is normally required to removethe selected nano-platelets from the suspension, and then furtherchemical washing is required to remove the surfactant and remainingfluids.

Whilst obtaining graphene from graphite has received particularattention, there are a number of other laminar materials which may bedelaminated to provide nanoscale platelets. The underlying process isone of comminution of a particulate feedstock, typically a mineral ore,under a shear regime that gives rise to delamination. Three basicmechanisms occur under high shear. A relevant reference is Ozcan-Taskin,N. G et al, “Effect of particle type on the mechanisms of breakup ofnanoscale particle clusters”, Chem Eng Res Des (2009), 10, 1016.

The first mechanism is erosion of the particulate feedstock. The firstmechanism is prevalent in rotor-stator devices, the well-knownSilverson™ mixer being one type. This mechanism gives rise to a bimodalparticle size distribution and has the advantage that processed productcan be more readily separated into the usually desired nano plateletsand the residual particulate feedstock due to the clear difference inphysical properties between the particles. The second mechanism isshattering, in which the particulate feedstock is entirely broken upinto a large number of very small particles. This is clearly the mostefficient and beneficial mechanism as it leads directly to the desiredproduct. However, this type of process is not currently available at alarge scale suitable for industrial production.

The third mechanism, rupture, has hitherto not been readily availablebut is now becoming available by means of impingement under highpressure of feedstock particles against an impact head as disclosed inco-pending patent application GB1518105.0. Such apparatus is capable ofoperating at an industrial scale but the resulting product is a mixtureof residual particulate feedstock, particles of an integral fraction ofthe feedstock size, such as a half, a quarter et cetera in the smallportion of nanoscale particles.

There is an opportunity to realise industrial scale production ofnanoscale particles if such products from an apparatus which uses thethird mechanism can be effectively separated, since processes that canbe carried out on the multiple kilogram scale, currently giving a lowyield, will still give a higher output than gram-scale processes ofhigher efficiency. There is therefore a need to separate nanoscaleparticles from an otherwise broad particle size distribution. Inparticular, there is a need to separate nanoscale platelets from amixture comprising a broad particle size distribution.

As mentioned, whilst obtaining graphene from graphite has receivedparticular attention there are a number of other laminar materials. Wheninvestigating delamination mechanisms under shear it would appear thatthe third mechanism mentioned above can be prevalent with several of thematerials even when using rotor-stator technology. There is therefore aparticular need for an industrial scale, and preferably continuous,separation method and apparatus to obtain nanoscale laminar materialfrom broad particle size distributions of such laminar materials.Relevant materials include the laminar forms of transition metaldichalcogenides; in this sense a chalcogenide being a sulphide, selenideor telluride of a transition metal such as tungsten or molybdenum. Alsorelevant are structurally similar materials such as boron nitride.

In addition, industrial scale processes and apparatus such as the onementioned in GB1518105.0 are conveniently and sometimes necessarilycarried out with large amounts of water or similar diluent as atransport medium. It would therefore be beneficial to have a separationmechanism so that a nanoscale platelet product could be available inmore concentrated form. This is particularly relevant to nanoscaleplatelets as their rate of sedimentation, if at all, is low andfiltration is not an effective mechanism for concentration.

A further requirement for effective industrial scale use is that anyprocedure should be relatively rapid. Similarly, a further requirementfor effective industrial scale use is that the apparatus and procedureunder which it operates should be capable of operating in continuousi.e. flow production mode rather than batch mode.

There exists therefore a need for a separation and/or concentrationprocess that can selectively remove submicron to micron scalenano-platelets, such as atomically thin 2-dimensional materialspreferably up to 100 nm in thickness, most preferably in the range oneatom thick to 30 layers, such as graphene and other submicron to micronscale laminar particles from a suspension as part of an industrial scaleproduction system.

The present invention seeks to overcome the problems in previoustechniques by providing a separation method for graphene that is rapid,scalable to industrial quantities and energy efficient.

SUMMARY OF THE INVENTION

The present invention in its various aspects is as set out in theappended claims.

The preferred form of continuous centrifuge is a disc stack separatorbut a disc bowl centrifuge may also be used. The disc stack centrifugeis preferably a conical plate centrifuge. Of the types of suchcentrifuges a nozzle-type centrifuge is preferred as this gives the mostefficient separation. The centrifuge is preferably operated in acontinuous mode in which both fine and coarse fractions are continuouslyremoved.

Preferred parameters of the preferred conical plate centrifuge (such asof the nozzle type) are:

A disc angle (relative to the axis of rotation) 30° to 50°, preferably35° to 40°. The number of discs is not critical and may be in the range10 to 100 discs. Disc separation is preferably from 1 mm to 20 mm, morepreferably from 5 mm to 10 mm, as this optimises size and throughputwhilst avoiding blockages for the feedstocks relevant to the presentinvention. A further parameter is the cone angle for discharge which ispreferably in the same angular range as the disc angle above.

The rate of rotation of the discs in continuous operation is determinedso as to provide centrifugal acceleration in the range between 2000 gand 25000 g, preferably between 4000 g and 18000 g between the inner andouter limits of the disc/cone. ‘g’ means acceleration in units of normalEarth gravity, or 9.8 ms⁻².

With a disc stack separator, unlike a conventional laboratorycentrifuge, simply providing a higher G force does not ensure better ormore efficient separation.

Counteracting the G force is a fluid flow velocity and this in the rangebetween 0.1 m/s to 0.00001 m/s, preferably between 0.01 m/s to 0.0001m/s between the inner and outer limits of the disc/cone.

The rotational speed of the disc stack separator can be selected to varyboth the centrifugal force which differentially separates the mixtureaccording to density, and also the turbulence present in different partsof the centrifuge which retains selected particles in suspension. Thedisc stack separator may be operable to rotate from 1000 RPM to 12000RPM. Preferably the separator is configured to operate at between 9000RPM and 9,600 RPM.

The effect of the rotation, in for example a disc stack with a radius of15 cm is to produce an acceleration on particles at the outer edge ofthe disc of between about 145,000 N/Kg and 151,000 N/Kg or 15,000 g. Theprocess is scalable by altering the rotational speed to produce the sameacceleration using a different size of disc.

The rate of input feed in conjunction with the disc stack separatordimensions affects the radial velocity of the fluid between the plates.In this case, we have selected a feed rate that provides an optimalvelocity of fluid. In particular, because the acceleration forces reduceas the suspension approaches the centre of the disc stack, while theradial velocity of the suspension increases, a balance can be obtainedbetween the Stokes velocity of the particles in suspension and themovement of the continuous phase.

The radial velocity of the fluid at the outer edge of the rotatingplates may be between 0.01 and 2 cm/s, preferably between 0.02 and 1cm/s.

To give an indication of the industrial nature of this process an inputfeed of fluid suspension is typically between 50 litres per hour and4000 litres per hour.

Due to the laminar nature of the nano-platelets a low flow rate has beenfound preferable, possibly because higher rates cause tumbling of theplatelets in turbulent flow and their effective size in the flow fieldof the apparatus varies as the plate angle varies relative to thedirection of flow. This feature clearly differentiates known uses ofdisc stack separators and their parameters of operation from the presentinvention since the small size of the platelets and their inter-atomicinteractions and the 2D nature of the platelets makes their behaviour ina shear field unpredictable.

The preferred fluid is water. The fluid of the present invention is aliquid. The fluid for use in the present invention is a suspension ofsolid particles in a liquid. A suspension of between 10 g/L and 200 g/Lof solids is preferred.

The ratio of the cone/disc outer diameter to the cone height ispreferably in the range 0.95 to 1.05.

For the purposes of this description, laminar solid particles areparticles of a material having a crystalline structure comprisingatomically thin layers, which can be separated by exfoliation (ordelamination) processes into atomically thin 2-dimensional materials. Anatomically thin 2-dimensional material may be defined as a materialhaving dimensions in a plane at least an order of magnitude (10×),preferably two orders of magnitude (100×) greater than the thickness ofthe particle, and the thickness of the particle being in the order ofsubmicron thickness, preferably up to 100 nm in thickness, mostpreferably between one atomic layer thick and 30 layers thick.

The most well-known of these materials is graphite, which can beexfoliated into the two-dimensional material graphene. Hexagonal boronnitride is another material which has been successfully exfoliated intomono and few atomic layer structures.

Another suitable material is molybdenum disulphide. Molybdenumdisulphide is a member of the group of transition metal dichalcogenides.Transition metal dichalcogenides (TMDC) are semiconductors of the typeMX₂, with M a transition metal atom (Mo, W, etc.) and X a chalcogen atom(S, Se, or Te). One layer of M atoms is sandwiched between two layers ofX atoms. Many of the TMDC group of compounds form layered crystalstructures, with separate layers bound together by weaker Van-der-Waalsforces, allowing them to be exfoliated into monolayers.

Niobium diselenide, vanadium telluride, manganese oxide and molybdenumtrioxide, are further materials which exhibit the ability to beexfoliated into few atomic layer platelets and can be separated in sizeusing the present invention.

The present invention provides

A method of continuously separating a solid suspension containingsubmicron thickness laminar solid particles into a submicron thicknessparticle fraction suspension and a residual particle fractionsuspension, the method comprising the steps of:

providing a continuous centrifuge apparatus;

providing a solid suspension of submicron thickness laminar solidparticles in a solid suspension;

separating the solid suspension in the apparatus;

wherein the solid suspension comprises the submicron thickness laminarsolid particles in a liquid continuous phase.

The present invention also provides a system for the continuousseparation of delaminated submicron thickness particles from asuspension of laminar particles, the system comprising:

a continuous centrifuge apparatus;

wherein the suspension of laminar particles comprises a solid suspensioncomprising submicron thickness solid particles in a liquid continuousphase;

the system being configured to continuously feed the first solidsuspension to the centrifuge which separates solid suspension into asuspension of submicron thickness laminar particles and a solidsuspension residue.

The solid suspension may have been produced using delamination apparatusfor exfoliation of the suspended solids, by feeding a precursor solidsuspension comprising supra-micron solid suspension of a laminarmaterial in a liquid continuous phase capable of being de-laminated intosubmicron thickness laminar solid particles;

where the delamination apparatus is configured to continuously receivethe precursor solid suspension and partially delaminate the laminarmaterial into submicron thickness particles comprising the first solidsuspension and; configured to continuously feed the resulting solidsuspension from the delamination apparatus to the centrifuge. Theresulting solid suspension will contain some submicron thicknessparticles in suspension, where the submicron thickness particles areatomically thin 2-dimensional materials, i.e. having a thickness lessthan one micron, preferably less than 100 nm, most preferably betweenone atom thick up to 30 layers. These particles may be referred to asnano-platelets. A portion of the original material may have beenincompletely delaminated, and will remain as supra-micron particles insuspension.

In some examples the delamination apparatus may be a high-pressurehomogeniser, such as described in pending patent applicationGB1518105.0. The delamination apparatus may be a high shear rotor-statormixer, a fixed geometry homogeniser or other known methods to exfoliatefew-layer nano-platelets from laminar material. The method of use of theapparatus of the present invention is very preferably uponbroad-spectrum particle size material such as produced by delaminationusing a high-pressure homogeniser. The method of use of the presentinvention gives a practical and industrial scale means to separate outatomically thin two-dimensional materials produced by such an apparatus.

Advantageously, the method or the system described may be used tocontinuously separate suspensions of partially delaminated graphite orhexagonal boron nitride.

Optionally, the method or system can be used to continuously separatesuspensions of molybdenum disulphide, tungsten diselenide or othertransition metal dichalcogenides. Transition metal dichalcogenides(TMDC) are semiconductors of the type MX₂, with M a transition metalatom (selected from Mo, W) and X a chalcogen atom (selected from S, Se,or Te). In these materials, it is understood that one layer of M atomsis sandwiched between two layers of X atoms.

Furthermore, it is envisaged that the method or the system may be usedto continuously separate suspensions of layered silicates, perovskites,niobates, layered metal oxides, metal halides and transition metal tri-or di-chalcogenides, graphite, boron nitride, molybdenum disulphide,tungsten diselenide or other transition metal dichalcogenides, graphyne,borophene, germanene, silicene, stanene, phosphorene, graphane,germanane, Bi₂Sr₂CaCu₂O_(x) or combinations thereof.

Preferably, the method or the system may be used in separating submicronthickness particles of graphite, boron nitride, molybdenum disulphide,molybdenum diselenide, molybdenum ditelluride, tungsten disulphide andtungsten diselenide. More preferably the present invention may be usedfor separating delaminated molybdenum disulphide, molybdenum diselenide,molybdenum ditelluride, tungsten disulphide and tungsten diselenide.Most preferably the present invention may be used to separate grapheneplatelets by size, most preferably graphene platelets as derived fromgraphite.

The invention provides a system and a method for continuously separatingsubmicron thickness particles, preferably in the particle thicknessrange of 1 to 100 nm. The submicron particles are preferably atomicallythin two-dimensional materials, such as platelets. For such materials,the particle size in the X and Y planes (i.e. as opposed to thickness inthe Z plane) may be up to 10 microns.

Preferably each of the suspensions is a solid suspension comprisingwater as the continuous phase.

Preferably each of the suspensions is stabilised by means of asurfactant.

More preferably the surfactant is sodium cholate.

Preferably the suspension is a suspension of graphite which has beenpartially delaminated into graphene.

In some embodiments, the continuous phase has a density greater thanwater but no more than the density of the suspended solid.

Preferably the continuous centrifuge apparatus is a disc stackcentrifuge. A disc stack centrifuge is a type of centrifuge that enablescontinuous separation of particles from suspension. A continuous flow ofsolid suspension may be introduced into the centrifuge. A stack ofinclined discs rotates with the suspension, increasing the surfacesettling area which speeds up the separation process. The densersediments collect at the periphery of the separator and may be releasedeither continuously or intermittently while the centrifuge continues tooperate, allowing the sediments to be reprocessed while the supernatantis extracted through a different path.

A traditional, non-disc stack laboratory type centrifuge would causesedimentation of the particles. The supernatant fluid would beextracted, and the sedimented particles would be removed from thecentrifuge and then redispersed in a solvent, after which repeatedcentrifugation would again cause sedimentation of some of thenano-platelets. This process may need to be repeated until the desiredseparation is achieved. A disc stack centrifuge process may continuouslyredisperse and separate sedimented nano-platelets. For example, a phaseenriched with mixed nano-platelets and graphite particles may becontinuously circulated through the disc stack centrifuge, becoming moreand more refined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through a typical disc stack centrifuge as usedin the present invention.

FIG. 2 shows a section through a typical disc stack centrifuge with thesolids discharge gap open.

FIG. 3 shows a chart of particle size distribution of a graphenedispersion produced by the current invention.

FIG. 4 shows a chart of particles size distribution of a boron nitridedispersion as produced by the current invention.

FIG. 5 shows the Raman spectra of a sampled of graphene produced by thecurrent invention.

FIG. 6 shows a chart of the particle size distribution of a molybdenumdisulphide suspension after the use of the method of the presentinvention.

FIG. 7 shows a chart of particle size distribution of a graphenedispersion produced by the present invention compared with a dispersionproduced by an alternative process.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 and FIG. 2 show a cross section through a disc stack centrifuge(100). Separation takes place inside a rotating vessel (110). Duringoperation, the vessel rotates about axis (170), generating centrifugalforce in a radial direction. The suspension is introduced to therotating centrifuge vessel (110) from the bottom via an inlet pipe(120), and is accelerated in the distributor (130), which may beconfigured to provide smooth acceleration of the input suspension.Leaving the distributor, the suspension enters the disc stack (140). Theseparation takes place between the discs, with the liquid continuousphase moving radially through the disc stack towards the centre. Duringthis movement, the suspended solids that are denser than the liquidcontinuous phase are differentially accelerated outwards in the oppositedirection to the liquid movement. When the liquid reaches the centre, itis discharged through the exit (150) where it is collected or may berecirculated for further centrifugation. The particles separated fromthe suspension move to the periphery (160).

These particles collect in the periphery where they may be discharged.The solid may be discharged by means of a gap (180) between the top(190) and bottom (200) of the rotating vessel at suitable pre-setintervals, the gap being opened by mechanical means for example ahydraulic system, or the gap can be set at a permanent suitable width toenable a continuous discharge of solids.

In the present invention, the disc stack centrifuge may be used toseparate graphene nano-platelets according to their thickness orparticle size. In some embodiments, the disc stack centrifuge can enablea sorting of the graphene nano-platelets according to the number ofatomic layers present.

In the present invention, the disc stack centrifuge may be used toseparate hexagonal boron nitride nano-platelets according to theirthickness or particle size.

In the present invention, the disc stack centrifuge may be used toseparate molybdenum disulphide nano-platelets according to theirthickness or particle size.

Molybdenum diselenide, molybdenum ditelluride, tungsten disulphide,tungsten diselenide are further materials which have shown some successwith the separation process described by the present invention.

For example, a disc stack centrifuge may be used to concentratenano-platelets having at least one common characteristic (e.g., sheetresistance, Raman spectra, number of atomic layers).

The present invention may exclude graphene and hexagonal boron nitride.

With a 2-dimensional material such as graphene the rate of separation ofparticles depends on both the effective density and the size of eachparticle, and as the particles are not spherical the separation rate mayalso depend on the orientation of the platelets.

FIG. 3 shows an example of the particle size distribution of a graphenesuspension, before and after the use of the present invention. Theparticle size was measured using a Malvern Mastersizer™. The solid line(500) shows the range of particle sizes in the first suspension beforethe disc stack centrifuge process was carried out. The peak has amaximum at approximately 13.5 μm. The dashed line (510) shows the rangeof particle sizes in the suspension after the disc stack centrifugeprocess. The peak has a maximum at approximately 0.05 μm. Thisdemonstrates that the disc stack centrifuge has surprisingly (?)separated the graphene nano-platelets very effectively compared withprior art techniques.

We have demonstrated that this technique using the equipment andconfiguration we have identified can separate nano-platelets that areparticularly suitable for end uses such as conductive inks, in acontinuous flow process that can process from 50 litres per hour to 4000litres per hour of suspension, compared with prior art techniques thatrequire many hours to process a few millilitres of suspension.

FIG. 4 shows an example of the particle size distribution of a boronnitride suspension before and after the use of the present invention.The solid line (520) shows the range of particle sizes in the firstsuspension before the disc stack centrifuge process was carried out. Thepeak has a maximum at approximately 0.5 μm. The dashed line (530) showsthe range of particle sizes in the suspension after the disc stackcentrifuge process. The peak has a maximum at approximately 0.26 μm.This demonstrates that the disc stack centrifuge has also separated theboron nitride nano-platelets very effectively.

FIG. 5 shows an example of Raman spectroscopy of a sample of graphenenano-platelets prepared using the present invention. The G peak (600),the D peak (610) and the 2D peak (620) are used to indicate the numberof defects and sheet thickness. In this case the ratio D/G=0.49 which ishigher than some samples obtained using other methods, and is perhapsconsistent with smaller flakes.

The symmetric shape of the 2D peak indicates that the sample has goodquality in terms of percentage of monolayer graphene.

The sheet resistance of this sample was 8.26 Ohm/Sq, which isadvantageously lower than many samples produced using prior arttechniques.

FIG. 6 shows an example of the particle size distribution of amolybdenum disulphide suspension after the use of the method of thepresent invention. The solid line shows the range of particle sizes inthe suspension after the disc stack centrifuge process. The peak has amaximum centered at approximately 0.72 μm. The material entering thedisc stack separator (not shown) had a mean particle size of 100 μm.This demonstrates that the disc stack centrifuge has also separated themolybdenum disulphide nano-platelets very effectively into a sub-micronfraction.

FIG. 7 shows a chart of particle size distribution of a graphenedispersion produced by the present invention compared with a dispersionproduced using a hydrocyclone. A hydrocyclone is another continuousseparation process that is used to separate particles in suspension intotwo streams according to particle size and/or mass. The dotted trace onthe chart shows the starting suspension which in this case is a graphenedispersion produced using a homogeniser. The peak particle size isaround 7 microns. The dashed trace shows the suspension resulting fromthe overflow of a hydrocyclone, which has a similar distribution to thestarting suspension. The solid line then shows the particle sizedistribution of the overflow from a disc stack separator, showing aclear reduction in peak particle size by volume, to around 0.1 micron.

The process of the invention allows the selection of desirableproperties of the material, which may be classified in the followingways.

As an example, the nano-platelets may be classified by the lateral sizeof each platelet. A range of methods have been used to give anindication of the particle size distribution of the graphenenano-platelets. This ranges from Transmission Electron Microscope (TEM)analysis for an “idealised” system of discrete particles to a laserscattering testing and sieve analysis for powders and suspensions ofnano-platelets that will be more indicative of the end product.

Lateral flake size may be measured by scanning or transmission electronmicroscopy. This technique comprises preparing a sampled of either thepowder or a dispersion of the material, producing an electron microscopeimage, and then measuring the perpendicular length and width of theflakes.

TEM samples are drop-casted onto holey carbon grids and allowed to dryat 60° C. for 72 hours under vacuum. Bright field and energy filteredTEM micrographs are taken at random locations across the grids, toensure a non-biased representation of the level of exfoliation.

The samples may be characterised using low resolution TEM. The aim ofthis is twofold: to assess the nature and quality of the exfoliatedflakes; and in some cases, to measure the lateral flake dimensions.Samples are prepared by drop-casting and imaging the grids on a Jeol2100™ TEM operating with an LaB6 electron gun at 200 kV.

The thickness of samples prepared in the same way can also be measuredusing atomic force microscopy (AFM). This technique can both provideestimates of lateral size as well as thickness of nano-platelets.

In practice electron microscope techniques are quite time-consuming toperform. Therefore, a quicker method of size classification is usedduring testing and production, which is compared to the electronmicroscope imagery for calibration.

Samples of the products have been analysed using a Malvern Mastersizer3000™. This uses the technique of laser diffraction to measure the sizeof particles, by measuring the intensity of light scattered as a laserbeam passes through a dispersed particulate sample. This data is thenanalysed to calculate the size of the particles that created thescattering pattern. The Malvern Mastersizer™ settings were“non-spherical particle mode”, using red and blue light, and water asthe medium. A stirrer was fitted to ensure the samples were uniform.

Sieve testing has been used to give an indicative size distribution byweight, which is analogous to the measurements obtained at the MalvernMastersizer™ by volume. The results are in a similar range to thatobtained in the Malvern Mastersizer™ which gives an independent crosscheck of the latter's measurements.

Two sieve sizes are used, a 150 μm and a 38 μm hole size. The sieves areweighed and stacked and then placed on the sieve shaker. 5.29 g ofpowder produced by drying the product of the present invention is addedin the top. An initial time of 5 minutes is used at full shaker capacity(3 mm/g). The material is then weighed and the sieves are reassembledwith the material still in the sieves. The sieves are then put back onthe shaker for another 5 minutes.

Raman spectroscopy is used to classify the quality of the platelets. Inthe case of graphene, the G peak, the D peak and the 2D peak are used toindicate the number of defects and sheet thickness. The ratio ofintensity of D/G bands is a measure of the defects present on graphenestructure. The G band is a result of in-plane vibrations of SP2 bondedcarbon atoms whereas the D band is due to out of plane vibrationsattributed to the presence of structural defects. All platelets haveedges, which are defects in the crystal structure and so the D band isnever zero as it would be in a perfect, infinite plane. If there aresome randomly distributed impurities or surface charges in the graphene,the G-peak can split into two peaks, G-peak (1583 cm-1) and D′-peak(1620 cm-1). The main reason is that the localized vibrational modes ofthe impurities can interact with the extended phonon modes of grapheneresulting in the observed splitting. The D peak usually appears around1350 cm-1 and the G peak usually appears around 1570 cm-1. The 2D peaksometimes labelled as G′ corresponds to the same vibrations as the Dband, but can be used to assess the number of atomic layers in a sample.Combination peaks D+D′ and D+D″ may appear around 2460 cm⁻¹ and 2930cm⁻¹.

The Raman spectra is measured using a Horiba XploRA™ Raman Microscope.Samples are supplied as a thin film on a filter membrane. After baselineremoval, the peaks D, G, D′, D+D′ and 2D are manually identified usingthe manufacturer's analysis software. The D/G ratio is calculated bydividing the peak intensity of the D peak by the peak intensity of the Gpeak. At least five spectra are analysed for each sample, and the D/Gand D/D′ ratio is calculated for each, which are then averaged.

In the present analysis, the interpretation of the 2D peak is not asstraightforward as in pure graphene. Due to a large number of flakes ineach sample, each flake gives a 2D band, the intensity and shape ofwhich is dependent on the number of layers and stacking of those layers.In single layer graphene, the 2D peak is a single peak with a 2D/G ratioaround 4, whereas for bilayer graphene the peak splits and the intensityreduces. The peak changes shape with each additional layer.

Without wishing to be bound by theory, when Raman spectroscopy iscarried out on a sample of nano-platelets with many overlapping flakes astraightforward characterization is not possible. In general, the 2Dpeak is less clearly defined than for single layer graphene, but awell-rounded peak is indicative of more single layer graphene in thesample than the uneven peaks caused by multilayer samples.

An important parameter is the sheet resistance, measured in ohms/square.The measurement using a four-point probe is carried out on a sample ofthe product applied to a substrate, such as a graphene film on a nylonmembrane.

The sample is weighed, and measurement is taken according toinstructions from the manufacturer of the four-point probe. In thiscase, we used a Jandel™ RM3000 set to 10 mA measuring current. Sixmeasurements are taken at different positions of each sample and anaverage taken. The measurement is normalized based on the measuredweight to an equivalent 30 mg sample.

For both Raman and Sheet resistance measurements, the samples wereprepared by the following steps:

Obtain a dispersion with nominal concentration of 100 g/L.

Obtain Nylon 66 membrane and plastic petri dish for storage.

Weigh nylon membrane in precision scales and note weight on petri dish.

Obtain vacuum pump and place in fume cupboard.

Place weighed membrane on glass filter ensuring the centre point of themembrane is aligned with the centre point of the glass filter.

Wet the membrane with a few ml of de-ionised water until membrane issoaked with water.

Place glass funnel on glass filter and clamp in place with metal clamp

Turn on vacuum pump, until nylon membrane rests firmly and uniformly onglass filter and all water is filtered through the membrane.

Using a micro-pipette measure 0.3 ml of graphene dispersion in avolumetric cylinder and top up dispersion with de-ionised water up to atotal volume of approximately 20 ml.

Turn vacuum pump on and pour dispersion in glass funnel.

When all water is filtered through pour 20 ml of de-ionised water infunnel and allow this to filter through.

Turn off vacuum pump.

Remove clasp and glass funnel.

Remove membrane from glass filter.

Place membrane on petri dish

Place petri dish in oven for at least 2 hours at 50° C.

An indication of the surface area of the product may be obtained by theBET (Brunauer-Emmett-Teller) gas adsorption method, supported by theelectron microscopy described above. Surface areas of the samples areanalysed by N2 physisorption at the liquid nitrogen temperature using aMicromeritics™ ASAP® 2020 instrument. Before the analysis, the samplesare de-gassed at 200° C. for 12 hours at a pressure lower than 10⁻³mmHg. The surface area of the samples is then calculated applying theBET equation to the collected data. The eventual presence of microporesis assessed by t-plot analysis. The pore size distributions andcumulative pore volumes for both adsorption and desorption branches arecalculated following the BJH (Barrett-Joyner-Halenda) method. All thecalculations are performed following the IUPAC recommendations.

In one example, using graphite particles delaminated into graphene, themeasurements before and after separation were taken using the techniquesdescribed above and produced the following results:

Sample L5443 Before separation After separation Sheet resistance Ω/Sq.11.7 5.9 Raman D/G  0.08 0.22 Modal particle size. 4.8 micron 0.12micron

Factors which may be controlled in the process to select the quality ofgraphene produced are as follows.

The loading of solid material in the liquid phase, which affects theviscosity of the suspension, and the productivity of delaminationprocess prior to the separation. Although the separation process may bemodelled as a Stokes settlement process, it is in fact far more complexas a suspension of mixed particles and platelets with a wide variationin size will not behave like a uniform suspension of spheres.

The solid content is preferably between 1% and 20% by weight, morepreferably between 1% and 10% by weight.

The range of particle size in the input suspension ranges from 0.01 μmto 100 μm as measured by particle size distribution analysis.

The range of thickness of the particles in the input suspension isbetween 0.2 nm and 100 μm, preferably less than 100 nm, most preferablyin the range one atom thick up to 30 layers thick.

The temperature of the suspension may affect the viscosity. Anadvantageous effect is that while it is known that the viscosity ofwater reduces with increasing temperature, the bulk viscosity of agraphene suspension has been discovered to increase with temperature.This enables the separation of graphene from a mixed suspension to befine-tuned by careful selection of the operating temperature during thecentrifugation stage in order to optimise the viscosity ratio betweenthe sediment and the supernatant. Advantageously we have found the mostefficient separation rate to take place at a temperature of between 5°C. and 50° C., for example between 20° C. and 40° C. preferably at 35°C.

The viscosity of the continuous phase may be between 0.0001 Pa·s and 10Pa·s. Preferably the viscosity is between 0.0001 Pa·s and 0.1 Pa·s. Mostpreferably the viscosity is between 0.0004 Pa·s and 0.001 Pa·s.

A surfactant may be added to the suspension which will also vary theviscosity and the stability of the suspension. The preferred surfactantis sodium cholate.

The density of the continuous liquid phase may be varied to improve thedegree of separation, although this will reduce the speed of separation.

In the case of nano-platelets, the effective density of the surfactantcoated platelet in suspension varies with the number of layers in theplatelet. For example, monolayer graphene in sodium cholate has aneffective density around 1.16 g/cc.

The density of the input suspension may be between 0.3 g/cc and 5 g/cc,preferably between 1 g/cc and 1.5 g/cc.

The density of the continuous phase liquid used in the suspension may bebetween 0.3 g/cc and 1.5 g/cc, preferably between 0.9 and 1.4 g/cc, mostpreferably 1.1 g/cc.

By selecting the appropriate combination of these factors, the discstack separator can be used in combination with an exfoliation processto select the desired properties of the nano-platelets in a continuousprocess, enabling efficient industrial scale production.

Measured Effectiveness of Separation.

Average Particle size Average Particle size (D4, 3) before (D4, 3) afterseparation Material separation (μm) (μm) Graphite/Graphene 13.5 0.05Hexagonal Boron 0.5 0.26 Nitride Molybdenum between 15 and 100 0.72disulphide Molybdenum 15 0.8 diselenide Molybdenum 50 Sub-micronditelluride Tungsten disulphide 20 0.8 Tungsten diselenide 25 Sub-micron

The last four sets of figures are subject to error due to small samplesize.

Other materials considered suitable for use in the present invention areniobium diselenide, vanadium telluride, manganese oxide and molybdenumtrioxide.

These sizes are as given by a Malvern Mastersizer™ using the standardsettings. These are indicative of the dimensions of the nano-platelets,using the sphere equivalent diameter. As we are operating on the limitof the operating range, using non-spherical samples, these figurespresent evidence of the successful size separation by the continuouscentrifuge, but are not indicative of the actual flake dimensions.However, the flake dimensions have been verified to be in a similarrange using electron microscopy and other methods described above.

The results and conditions in this document are taken at 25° C. unlessmentioned otherwise.

The invention claimed is:
 1. A method of continuously separating a solidsuspension containing submicron thickness laminar solid particles into asubmicron thickness particle fraction suspension and a residual particlefraction suspension, the method comprising the steps of: providing acontinuous centrifuge apparatus; providing the solid suspensioncontaining the submicron thickness laminar solid particles; separatingthe solid suspension in the continuous centrifuge apparatus; wherein thesolid suspension comprises the submicron thickness laminar solidparticles in a liquid continuous phase; wherein the continuouscentrifuge apparatus is a disc stack centrifuge; and wherein thesubmicron thickness laminar solid particles comprise particles of amaterial having a crystalline structure comprising atomically thinlayers, which have been partially delaminated into atomically thinnano-platelets.
 2. The method of claim 1, wherein the submicronthickness laminar solid particles comprise particles of partiallydelaminated graphite, hexagonal boron nitride, molybdenum disulphide,tungsten diselenide or other transition metal dichalcogenides.
 3. Themethod of claim 1, wherein the submicron thickness laminar solidparticles comprise particles of partially delaminated graphite orhexagonal boron nitride.
 4. The method of claim 1, wherein the submicronthickness laminar solid particles comprise particles of partiallydelaminated molybdenum disulphide, molybdenum diselenide, molybdenumditelluride, tungsten disulphide and tungsten diselenide.
 5. The methodof claim 1, wherein the submicron thickness laminar solid particles arein the particle size range of 1 to 100 nm.
 6. The method of claim 1, inwhich the solid suspension comprises water in the liquid continuousphase.
 7. A system for the continuous separation of delaminatedsubmicron thickness particles from a suspension of laminar particles,the system comprising: a continuous centrifuge apparatus; wherein thesuspension of laminar particles comprises a solid suspension comprisingsubmicron thickness solid particles in a liquid continuous phase;wherein the continuous centrifuge apparatus is a disc stack centrifuge;and wherein the submicron thickness solid particles comprise particlesof a material having a crystalline structure comprising atomically thinlayers, which have been partially delaminated into atomically thinnano-platelets; the system being configured to continuously feed thesolid suspension to the continuous centrifuge apparatus which separatesthe solid suspension into a suspension of submicron thickness laminarparticles and a solid suspension residue.
 8. The system of claim 7,wherein the suspension is a suspension of partially delaminatedgraphite, hexagonal boron nitride, molybdenum disulphide, molybdenumdiselenide, molybdenum ditelluride, tungsten disulphide and tungstendiselenide.
 9. The system of claim 7, wherein the submicron thicknesssolid particles comprise particles of partially delaminated graphite orhexagonal boron nitride.
 10. The system of claim 7, wherein thesubmicron thickness solid particles comprise particles of partiallydelaminated molybdenum disulphide, tungsten diselenide or othertransition metal dichalcogenides.
 11. The system of claim 7, in whichthe suspension of laminar particles is a solid suspension in water asthe continuous phase.